The present invention relates generally to radio receivers and associated antennas, and more particularly to a method and apparatus for automatically and adaptively tuning a radio receiver antenna.
Wristwatch paging radio receivers are known in the art, as illustrated by U.S. Pat. No. 4,713,808 issued to Gaskill et al. Such receivers often use antennas disposed in a band looped around the user's wrist or neck. The impedance of such an antenna is strongly dependent on many factors, including band length, degree of parasitic coupling to the user's body and other objects, and the frequency at which the receiver operates. As implemented in a frequency agile wristwatch radio signal receiving device, the impedance of an antenna provided as the wristband of the watch suffers particularly from adjustment in band size, i.e., to match the user's wrist over a wide range of frequencies, whether or not the device is being worn by the user, whether or not the band clasp is open or closed, the watch case material and construction and its effect on antenna capacitance, and parametric variations such as original component variation according to manufacturer tolerance in components or replacement component variation relative to device specifications.
Wristband antennas are relatively small with respect to wave length and typically have a relatively high Q. Even a slight change in any of the above-referenced impedance factors can result in a large change in tuned frequency. Furthermore, the small sized antenna provides a relatively small signal to the attached receiver. A tuning/impedance matching circuit must be used to closely match and tune the antenna to the receiver to insure optimum power transfer therebetween.
WIPO patent publication, 88/05213, discloses a wristwatch receiver embodying a variable matching circuit. In the disclosed system, the matching circuit includes a variable capacitor accessed by opening a cover element. Once exposed, the circuit is tuned with a screwdriver. The WIPO disclosure proposes adjusting the capacitor by using an auxiliary RF signal generator located near the receiver to radiate a signal at the desired reception frequency. The receiver displays the relative strength of the received signal on an LCD display positioned beneath the removal cover. With the RF signal generator activated, the user can adjust the variable matching capacitor until the display indicates that a maximum signal is being received.
The tuning procedure for the system disclosed in this WIPO publication requires manual intervention. It requires that a cover be removed from the watch and a micro miniature capacitor be adjusted with a screwdriver. The receiver may become detuned as soon as it is moved, whether off the user's wrist or simply towards or away from the user's body. Each such movement changes the antenna's parasitic coupling to the surrounding materials and thereby changes it resonant frequency which requires another tuning process.
A related WIPO publication, 88/05214, discloses a second wristwatch receiver embodying a variable tuning circuit. In this second system, an automatic tuning circuit within the wristwatch receiver is activated in response to reception of a tuning mode signal on the frequency being received. This tuning mode signal can be part of the system synchronization signal, an independent selective call signal decodable by an entire group of receivers or a secondary signal decoded after a receiver's individual selective call address has been decoded. The tuning operation can also be manually initiated by a user programmable switch.
Tuning results from adjustment in a varactor tuning element controllably biased by a microcomputer through a D-A converter. The biased voltage is increased, apparently in uniform steps, until a drop is noted in the level of the received signal. The biased voltage that was applied immediately preceding this drop is then selected as the proper tune voltage.
The magnitude of the received signal in this second system is determined by comparing it against four analog levels. If, in a given situation, all of the levels are exceeded, the apparatus must reduce the gain of the receiver until the received signal falls between the minimum and maximum comparison levels. Similarly, if the received signal is below all four levels, the apparatus must increase the gain of the receiver. This arrangement slows the tuning operation and renders the tuning system in operative in areas of very high or very low signal strength, where the receiver cannot bring the received signal within the range of the comparison levels.
The received signal strength is presumed under this system to have one local maximum as the varactor biased voltage increases. Consequently, as soon as a drop in received signal is noted, the system concludes that the optimum tuning condition has been determined, when in some cases the drop in signal strength could be only a local maximum and far below the true maximum signal at optimum antenna tuning. More likely, the received signal will have several local maxima, many of them spurious due to unpredictable effects such as noise and movement of the antenna during the tuning operation.
This system also requires that a tuning command signal be transmitted with the paging signals. The disclosure specifies that an unmodulated carrier should be transmitted for approximately 200 milliseconds following the command signal to permit the tuning apparatus to tune. This requires a redesigning of the paging protocol and uses time that could otherwise be used for message transmission. As may be appreciated, in the context of a time multiplexed paging protocol, such tuning time represents a degradation in overall information band width. The more often the tuning command signal is transmitted, the more time is diverted from message transmission.
If a receiver according to this second WIPO disclosure is moved between tuning intervals, it may be operated in an untuned state until another tune command is received. A receiver operated in an untuned state may miss a message.
U.S. Pat. No. 5,136,719 issued Aug. 4, 1992 to Gaskill et al shows an automatic antenna tuning method and apparatus. In accordance with the Gaskill method of automatic antenna tuning, prior to receiving each time multiplexed paging data packet the paging device automatically executes an antenna tuning procedure. The antenna includes a tuning terminal whereat variation in voltage causes variation in antenna impedance-tuning. By use of a D-A converter, the paging receiver controller steps through the entire antenna tuning voltage range at given magnitude voltage increments or steps. The controller monitors a received signal strength indicator, e.g., AGC signal, to detect the occurrence of a peak received signal strength condition and associate that condition with a given step in the tuning voltage range traversal. Given the antenna tuning voltage associated with maximum signal strength, the antenna is tuned using that voltage in time to receive and decode the data packet under optimum antenna tuning conditions.
One important parameter in the scanning of the entire antenna tuning voltage range under the Gaskill method is the number of steps taken, and therefore the magnitude of voltage increments between steps. The smaller the increments in voltage between each step, the more accurately the optimum tuning condition can be identified. If the voltage increment between steps is too large, the peak in the received signal strength indicator may be missed and optimum tuning conditions not achieved. Using a large number of steps, and, therefore, a small voltage increment between steps, requires greater processing time for the tuning cycle. Accordingly, there is a tradeoff between the time required to execute a tuning cycle and the accuracy achieved in that tuning cycle. The more time permitted for an antenna tuning cycle, the more accurate the antenna tuning. The faster the tuning cycle, the less accurate the antenna tuning. As may be appreciated, in a time multiplexed data packet delivery system it is desirable to minimize the time required to execute any procedure associated with receiving a packet. Thus, overall system requirements may impose certain time limitations on the antenna tuning procedure, and therefore impose limitations on the accuracy of the antenna tuning procedure as set forth under the Gaskill method.
The method of antenna tuning proposed under the above-noted Gaskill method suffers from an inability to compensate for or adapt to certain conditions. For example, the Gaskill method must select a fixed antenna tuning control voltage range based upon knowledge of specific watch conditions and component characteristics, e.g., knowledge of a specific varactor curve. If the actual conditions under the which the watch operates, or if substitute components are used having different characteristics, the selected antenna tuning control voltage range may be inappropriate, i.e., too big or too small, to detect and utilize optimum antenna tuning control voltage levels. Thus, the Gaskill method of antenna tuning lacks any adaptive capability with respect to change in device environment or component selection.
Thus, while the Gaskill method of automatic tuning provides great improvement in the context of time multiplexed paging protocols and associated radio receivers, it is desirable that the tuning cycle used in such a time multiplexed protocol be minimized with respect to time required for execution, yet retain high reliability and precision in accurately identifying the optimum antenna tuning conditions. The subject matter of the present invention provides such an improvement to the Gaskill method of automatic antenna tuning.